Schema validation failed after downgrade – can broker auto‑rollback topic schema version?

Direct answer

No, the Pulsar broker cannot automatically roll back a topic’s schema version when a validation error occurs; it simply rejects the produce request and leaves the schema at the new version.

Why auto‑rollback is not supported

The broker’s schema handling is immutable after a successful upgrade; on a mismatch it returns “Schema validation failed: expected version X, found Y” and aborts the request. There is no configuration flag or API that triggers a rollback, and older Pulsar releases lack any built‑in rollback capability.

Steps to handle a downgrade

1. Ensure the producer uses a schema that is backward compatible with the current version, or downgrade the producer before the broker rejects the message.
2. If the consumer must continue using the previous version, manually set the topic schema to the older version via the admin API (e.g., setTopicSchema(topic, oldSchema)).
3. Publish a test message with the older schema and confirm the consumer can deserialize it without throwing a DeserializationException.

Verification

Query the topic’s schema version with the admin API (getTopicSchema(topic)) and verify that the version field matches the rolled‑back value. Then, publish a message using the older schema and check that the consumer processes it without exception, confirming that message loss is avoided only when the exception is caught.